The purpose of this research is to develop a measure of language comprehension for 12 to 20 month old infants using the visual preference technique and to evaluate the relation between this measure and parental report. The relation between experimental and parental report in language production is generally strong but a similarly strong relation has not been obtained between experimental and parental report measures of comprehension. The proposed measure has implications for clinical language assessment. Early assessment of language delay is crucial to undertaking appropriate intervention yet, often, language delay is not identified until deficits in production are observed, as late as three to four years of age. A comprehensive, experimental measure of receptive vocabulary that spans the transition from receptive to expressive language across the first 24-30 months of life does not exist. The present research will develop this measure, based on a sensitive visual preference technique, and will make the following contributions to the literature: 1. The inclusion of vocabulary items based on frequency data from the CDI. 2. The assessment of the convergent validity of parental report and visual preference measures of comprehension. 3. The longitudinal assessment of the predictive validity of the measures.